Engineering News

Viennese Engineers Develop Inflatable Concrete

Inflatable concrete promises to radically reduce the amount of scaffolding material required during the construction process.

Researchers from the Vienna University of Technology have successfully tested a completely new way of building concrete structures which could render the need for complex, supporting timber structures obsolete.

By bending concrete with an air cushion, the team has successfully built an experimental concrete dome. A flat concrete slab hardens on the ground, and then an air cushion below the plate is inflated, bending the concrete and quickly forming a sustainable shell.

“It is similar to an orange peel, which is cut at regular intervals and then flattened out on the table”, said Professor Johann Kollegger. “We do it the other way around, starting with a flat surface and then bending it to a shell.”

The flat slab is created using standard concrete but it is imperative that the accuracy of the geometric shape is spot on. Divided like an orange into several segments, wedge-shaped spaces punctuate these pieces, so that when the structure is bent they come together perfectly. They wait for the slab to harden and then inflate the air cushion below.

This cushion comprises two plastic sheets welded together. As it inflates, a steel cable is tightened around the concrete segments, lifting it from the centre and pushing it together from outside.

The concrete segments are connected by metal beams to ensure they move together in perfect synchronicity. For the test dome, which was raised to a height of 2.9 metres, the process took around two hours.

Although tiny cracks appear when the concrete is bent, this does not compromise the stability of the shell.

“We can see that in old stone arches”, said Kollegger. “If the shape is right, each stone holds the others in place and the construction is stable.”

To negate the unsightly cracks, the structure is simply plastered.

“We decided not just to create a simple, rotationally symmetric shape”, said fellow researcher Benjamin Kromoser. “Our building is a bit drawn-out, it cannot be described in simple geometric terms. We wanted to show that using our technology, even complex free-form structures can be created.”

The researchers believe that carefully designing the shape of the concrete slab and air cushion will give architects the ultimate freedom and flexibility to create a wide variety of shapes.

“Building shells with a diameter of 50 metres is no problem with this technique”, said Johann Kollegger. “The hardest challenge is creating complicated shapes with a very small radius of curvature.”

Different kinds of reinforcement were tested in the university’s laboratory to see how strongly they could be bent with a radius of curvature of just three metres.

Kromoser estimates construction costs may be cut by half or more with the additional benefit of shorter programmes and the need for fewer resources as the supporting timber structure is no longer needed.

The Austrian Federal Railways (OEBB-Infrastruktur AG) has commissioned the first design project based on this method for a deer pass over two high speed rail tracks in Carinthia.